Boron nitride nanotubes (BNNTs) have exceptional thermal stability, thermal conductivity, mechanical properties, neutron radiation shielding, and piezoelectricity. Due to their multifunctional properties, BNNTs are potential candidates for sensory materials in harsh environments. Brittleness and non-conformity of conventional piezoelectric ceramics have limited their broad applications. Flexible and ultra-light piezoelectric sensors based on BNNTs could be an alternative solution in high temperature, high radiation, high shock, and severe vibration environments. In this study, BNNTPolyurethane (PU) composites were fabricated and their converse piezoelectric constant of d<sub>33</sub> was assessed using a laser Doppler vibrometer (LDV). This study demonstrated that BNNT could be an excellent piezoelectric nanofiller for flexible sensor applications.

Technique with the capability of detecting and localizing damage of structures using naturally operating environments can provide a possibility of developing more efficient and simpler structural health monitoring systems. This passive sensing technique would eliminate the need of active actuation which requires power either from battery or ambients to generate controlled excitation source. In a recent study, self-Green’s functions (GF) were reconstructed using auto-correlation (AC), combined with a damage index by comparing the differences in GFs between damaged and pristine metallic panels to locate the damage. In this paper, random decrement (RD) technique is proposed to reconstruct GF with computational efficiency. While the RD has been widely used for damage detection and structure parameter extraction in civil structures, in the frequency usually below 1 kHz; this study explores using RD up to 15 kHz for transient wave reconstruction and then damage localization. The concept is first validated through simulation for a plate structure, and the results show that the reconstructed self-Green’s function match well with the one from the auto-correlation technique after approximately 10,000 averages of the RD signatures.

Passive sensing technique may eliminate the need of expending power from actuators and thus provide a means of developing a compact and simple structural health monitoring system. More importantly, it may provide a solution for monitoring the aircraft subjected to environmental loading from air flow during operation. In this paper, a non-contact auto-correlation based technique is exploited as a feasibility study for passive sensing application to detect damage and isolate the damage location. Its theoretical basis bears some resemblance to reconstructing Green’s function from diffusive wavefield through cross-correlation. Localized high pressure air from air compressor are randomly and continuously applied on the one side surface of the aluminum panels through the air blow gun. A laser Doppler vibrometer (LDV) was used to scan a 90 mm × 90 mm area to create a 6 × 6 2D-array signals from the opposite side of the panels. The scanned signals were auto-correlated to reconstruct a “selfimpulse response” (or Green’s function). The premise for stably reconstructing the accurate Green’s function requires long sensing times. For a 609.6 mm × 609.6 mm flat aluminum panel, the sensing times roughly at least four seconds is sufficient to establish converged Green’s function through correlation. For the integral stiffened aluminum panel, the geometrical features of the panel expedite the formation of the diffusive wavefield and thus shorten the sensing times. The damage is simulated by gluing a magnet onto the panels. Reconstructed Green’s functions (RGFs) are used for damage detection and damage isolation based on an imaging condition with mean square deviation of the RGFs from the pristine and the damaged structure and the results are shown in color maps. The auto-correlation based technique is shown to consistently detect the simulated damage, image and isolate the damage in the structure subjected to high pressure air excitation. This technique may be transformed into passive sensing applied on the aircraft during operation.

The barely visible impact damages reduce the strength of composite structures significantly; however, they are difficult to be detected during regular visual inspection. A guided wave based damage imaging condition method is developed and applied on a curved composite panel, which is a part of an aileron from a retired Boeing C-17 Globemaster III. Ultrasonic guided waves are excited by a piezoelectric transducer (PZT) and then captured by a laser Doppler vibrometer (LDV). The wavefield images are constructed by measuring the out-of-plane velocity point by point within interrogation region, and the anomalies at the damage area can be observed with naked eye. The discontinuities of material properties leads to the change of wavenumber while the wave propagating through the damaged area. These differences in wavenumber can be observed by deriving instantaneous wave vector via Riesz transform (RT), and then be shown and highlighted with the proposed imaging condition named wavenumber index (WI). RT can be introduced as a two-dimensional (2-D) generalization of Hilbert transform (HT) to derive instantaneous phases, amplitudes, orientations of a guided-wave field. WI employs the instantaneous wave vector and weighted instantaneous wave energy computed from the instantaneous amplitudes, yielding high sensitivity and sharp damage image with computational efficiency. The BVID of the composite structure becomes therefore “visible” with the developed technique.

Oil lubrication plays an important role in a variety of mechanical equipment. The traditional purification method is difficult to remove the tiny impurity size of 5-15 μm. Three different types of the transducers and its preparation methods were used in the experiment. The phenomenon that the impurity particles in viscous fluid by the acoustic radiation force was moved the wave node position and focused on the center line was observed by the super-depth microscope. The influence factors of the produced SSAW, particle force condition and movement track were analyzed. The experimental results show that the interdigital transducer can be used to generate SSAW, so as to achieve the separation effect of oil and suspended particles.

In this paper a method for damage detection in beam structures using high-speed camera is presented. Traditional methods of damage detection in structures typically involve contact (i.e., piezoelectric sensor or accelerometer) or non-contact sensors (i.e., laser vibrometer) which can be costly and time consuming to inspect an entire structure. With the popularity of the digital camera and the development of computer vision technology, video cameras offer a viable capability of measurement including higher spatial resolution, remote sensing and low-cost. In the study, a damage detection method based on the high-speed camera was proposed. The system setup comprises a high-speed camera and a line-laser which can capture the out-of-plane displacement of a cantilever beam. The cantilever beam with an artificial crack was excited and the vibration process was recorded by the camera. A methodology called motion magnification, which can amplify subtle motions in a video is used for modal identification of the beam. A finite element model was used for validation of the proposed method. Suggestions for applications of this methodology and challenges in future work will be discussed.

A vision-based damage detection technique was proposed for the identification of damages in composite honeycomb structures. The motion above the damage area extracted from the wave field image with the developed image decomposition and image signal processing method reveals rich information to determine damage severity.
The standing wave prevailed at its resonant frequencies above the barely visible impact damage (BVID) on the surface of a CFRP/honeycomb composite sandwich plate, which was excited by a Q-Switch Nd:YAG pulse laser system for generating a broad-band guided wave, and the wavefield was captured by a Laser Doppler Vibrometer (LDV). With the developed image processing technique, the wavefield image that contains incident waves, reflected waves and standing waves could be separated from different wavenumber vectors and propagating directions. Phases, orientations and resonant frequencies derived from the separated standing wave were taken advantage of, for either emphasizing or magnifying the motion and illustrating the modal behavior on the damage surface. The barely visible impact damage (BVID) of the composite structure was therefore “visible” with the developed technique.

Lamb wave dispersion curves for isotropic plates are extracted from measured sensor data by matrix pencil (MP) method. A piezoelectric wafer emits a linear chirp signal as broadband excitation to generate Lamb waves in isotropic plates. The propagating waves are measured at discrete locations along a wave ray direction with a sensor 1-D laser Doppler vibrometer (LDV). The out-of-plane velocities are first Fourier transformed into either space-frequency <i>x-&omega;</i> domain or wavenumber-time <i>k-t</i> domain. The matrix pencil method is then employed to extract the dispersion curves for various wave modes simultaneously. In addition, the phase and group velocity dispersion curves are deduced by the relation between wavenumber and frequency. In this research, the inspections for dispersion relations on isotropic plates are demonstrated and compared by two-dimensional Fourier transform (2D-FFT) and MP method. The results are confirmed by theoretical curves computed numerically. It has demonstrated that the MP method is robust in recognining/differentiating different wave modes, including higher order ones.

A matching pursuit (MP) algorithm is effective tool to decompose the overlapped wave packets in a signal so that each wave mode can be identified. For the successful separations of the wave packets, an atom function should be properly designed, that can well resemble the physical features of the signal of interest. In this paper, a novel atom function for the MP algorithm is proposed based on the wave propagating model due to an excitation of a Hann-windowed toneburst signal, which performs very accurately compared to the MP algorithm with the existing Gaussian-type atom functions. The decomposed wave packets, including the directly scattered wave from damage as well as the reverberant waves from the free edges of the plate, via the MP method are employed in the damage imaging algorithm, highlighting the damaged location with higher intensity than the conventional algorithm utilizing only a direct reflected wave. The proposed approach is verified from the experiment where four piezoelectric wafers can accurately identify the damage location in a plate.

This article presents 3-degree-of-freedom theoretical modeling and analysis of a low-frequency vibration energy harvester based on diamagnetic levitation. In recent years, although much attention has been placed on vibration energy harvesting technologies, few harvesters still can operate efficiently at extremely low frequencies in spite of large potential demand in the field of structural health monitoring and wearable applications. As one of the earliest works, Liu, Yuan and Palagummi proposed vertical and horizontal diamagnetic levitation systems as vibration energy harvesters with low resonant frequencies. This study aims to pursue further improvement along this direction, in terms of expanding maximum amplitude and enhancing the flexibility of the operation direction for broader application fields by introducing a new topology of the levitation system.

This article identifies and studies key parameters that characterize a horizontal diamagnetic levitation (HDL) mechanism based low frequency vibration energy harvester with the aim of enhancing performance metrics such as efficiency and volume figure of merit (FoM<sub>v</sub>). The HDL mechanism comprises of three permanent magnets and two diamagnetic plates. Two of the magnets, aka lifting magnets, are placed co-axially at a distance such that each attract a centrally located magnet, aka floating magnet, to balance its weight. This floating magnet is flanked closely by two diamagnetic plates which stabilize the levitation in the axial direction. The influence of the geometry of the floating magnet, the lifting magnet and the diamagnetic plate are parametrically studied to quantify their effects on the size, stability of the levitation mechanism and the resonant frequency of the floating magnet. For vibration energy harvesting using the HDL mechanism, a coil geometry and eddy current damping are critically discussed. Based on the analysis, an efficient experimental system is setup which showed a softening frequency response with an average system efficiency of 25.8% and a FoM<sub>v</sub> of 0.23% when excited at a root mean square acceleration of 0.0546 m/s<sup>2</sup> and at frequency of 1.9 Hz.

A two-dimensional (2-D) non-contact areal scan system was developed to image and quantify impact damage in a composite plate using an enhanced zero-lag cross-correlation reverse-time migration (E-CCRTM) technique. The system comprises a single piezoelectric actuator mounted on the composite plate and a laser Doppler vibrometer (LDV) for scanning a region to capture the scattered wavefield in the vicinity of the PZT. The proposed damage imaging technique takes into account the amplitude, phase, geometric spreading, and all of the frequency content of the Lamb waves propagating in the plate; thus, the reflectivity coefficients of the delamination can be calculated and potentially related to damage severity. Comparisons are made in terms of damage imaging quality between 2-D areal scans and linear scans as well as between the proposed and existing imaging conditions. The experimental results show that the 2-D E-CCRTM performs robustly when imaging and quantifying impact damage in large-scale composites using a single PZT actuator with a nearby areal scan using LDV.

Ultrasonic guided waves are one of the most prominent tools for SHM in plate-like structure. However, complex propagation characteristics of guided waves as well as traditional contact ultrasonic transducers limit its application in the practical damage detection. Scanning Laser Doppler vibrometer (SLDV) technology is an effective non-contact method to obtain ultrasonic guided wavefield with ultra-high spatial resolution. Based on abundant wavefield data, wavenumber imaging algorithms are capable of not only damage location, but also assessment of damage characteristics such as size and shape. In this work, we adopt local wavenumber analysis method for horizontal crack detection in platelike structure. Instead of using SLDV in experiment, 3D finite element numerical method is adopted to obtain full ultrasonic guided wavefield data. Since the horizontal cracks result in decrease of local thickness, the wavenumber in corresponding area shows significant increase, which is used as indicators for crack imaging. The effects of different damage shapes, depths and spatial window sizes on imaging are also discussed. Numerical simulation results and imaging algorithm laid the foundation for the method applied in experiment and practice.

The spectral finite element method (SFEM) is developed to predict guided ultrasonic waves in the surface-bonded piezoelectric wafer and beam structure. The Timoshenko beam theory, the Euler-Bernoulli beam theory and linear piezoelectricity are used to model the base beam and electric-mechanical behavior of the piezoelectric wafer respectively. Using Hamilton’s principle, the governing equations are obtained in the time domain, and then the SFEM are formulated from coupled differential equations of motion transformed into the frequency domain via the discrete Fourier transform. The SFEM is used to analyze the dispersion characteristics, mode conversion of guided waves and the interaction of waves and notch. The high accuracy of the present SFEM is verified by comparing with the finite element method results.

This article investigates a horizontal diamagnetic levitation (HDL) system for vibration energy harvesting. In this configuration, two large magnets, alias lifting magnets, are arranged co-axially at a distance such that in between them a magnet, alias floating magnet, is passively levitated at a laterally offset equilibrium position. The levitation is stabilized in the horizontal direction by two diamagnetic plates made of pyrolytic graphite placed on each side of the floating magnet. This HDL configuration permits large amplitude vibration of the floating magnet and exploits the ability to tailor the geometry to meet specific applications due to its frequency tuning capability. Theoretical modeling techniques are discussed followed by an experimental setup to validate it. At an input root mean square (RMS) acceleration of 0.0434 m/s<sup>2</sup> (0.0044 g<sub>rms</sub>) and at a resonant frequency of 1.2 Hz, the prototype generated a RMS power of 3.6 &mu;W with an average system efficiency of 1.93%. Followed by the validation, parametric studies on the geometry of the components are undertaken to show that with the optimized parameters the efficiency can be further enhanced.

A flexoelectric bridge-structured microphone using bulk barium strontium titanate (Ba<sub>0.65</sub>Sr<sub>0.3</sub>5TiO<sub>3</sub> or BST) ceramic was investigated in this study. The flexoelectric microphone was installed in an anechoic box and exposed to the sound pressure emitted from a loud speaker. Charge sensitivity of the flexoelectric microphone was measured and calibrated using a reference microphone. The 1.5 mm&times;768 &mu;m&times;50 &mu;m micro-machined bridge-structured flexoelectric microphone has a sensitivity of 0.92 pC/Pa, while its resonance frequency was calculated to be 98.67 kHz. The analytical and experimental results show that the flexoelectric microphone has both high sensitivity and broad bandwidth, indicating that flexoelectric microphones are potential candidates for many applications.

This paper presents a new method for monitoring and characterizing cracks using Ba<sub>0.64</sub>Sr<sub>0.36</sub>TiO<sub>3</sub> flexoelectric strain gradient sensors. Firstly, strain gradient field around the mixed mode asymptotic crack tip was analyzed, followed by the derivation of induced flexoelectric polarization in the strain gradient sensors attached in the vicinity of a crack tip. It was found that the flexoelectric polarization of the sensor can be expressed as a function of the stress intensity factors of crack and relative coordinates between the sensor and crack. Given the information of the crack size, further analysis demonstrates that the location of the crack can be traced through the calculation based on flexoelectric outputs of the distributed sensors. A specimen with Mode-I crack was then prepared with two strain gradient sensors (4.7 mm &times; 0.9 mm &times; 0.3 mm) attached close to the crack tip to verify the analytical model for detection of cracks. The experimental results yield accurate location of the crack, confirming that flexoelectric strain gradient sensing can be a good avenue for monitoring cracks.

A curvature sensor based on flexoelectricity using Ba0.64Sr0.36TiO<sub>3</sub> (BST) material is proposed and developed in this paper. The working principle of the sensor is based on the flexoelectricity, exhibiting coupling between mechanical strain gradient and electric polarization. A BST curvature sensor is lab prepared using a conventional solid state processing method. The curvature sensing is demonstrated in four point bending tests of the beam under harmonic loads. BST sensors are attached on both side surfaces of an aluminum beam, located symmetrically with respect to its neutral axis. Analyses have shown that the epoxy bonding layer plays a critical role for curvature transfer. Consequently a shear lag effect is taken into account for extracting actual curvature from the sensor measurement. Experimental results demonstrated good linearity from the charge outputs under the frequencies tests and showed a sensor sensitivity of 30.78pC&bull;m in comparison with 32.48pC&bull;m from theoretical prediction. The BST sensor provides a direct curvature measure instead of using traditional strain gage through interpolation and may offer an optional avenue for on-line and in-situ structural health monitoring.

This work presents guided wave generation, sensing, and damage detection in metallic plates using in-plane shear (<i>d</i><sub>36</sub> type) piezoelectric wafers as actuators and sensors. The conventional Lead zirconate titanate (PZT) based on induced in-plane normal strain (<i>d</i><sub>31</sub> type) has been widely used to excite and receive guided wave in plates, pipes or thin-walled structures. The <i>d</i><sub>36</sub> type of piezoelectric wafers however induces in-plane (or called face) shear deformation in the plane normal to its polarization direction. This form of electromechanical coupling generates more significant shear horizontal waves in certain wave propagation directions, whose amplitudes are much greater than those of Lamb waves. In this paper, an analysis of shear horizontal (SH) waves generated using in-plane shear electromechanical coupling is firstly presented, followed by a multiphysics finite element analysis for comparison purpose. Voltage responses of both conventional <i>d</i><sub>31</sub> and new <i>d</i><sub>36</sub> sensors are obtained for comparison purpose. Results indicate this type of wafers has potential for simply providing quantitative estimation of damage in structural health monitoring.

In this paper, a method to focus flexural Lamb waves to a local area by mounting elastic metamaterials (EMMs) on the
surface of the plate is proposed. The EMM consists of silicon rubber and lead connected in series bonded vertically on
an aluminum plate. A simplified effective mass-“spring”-mass model is used to study the EMM plate. The frequency-dependent
effective mass density of the EMM plate is determined with the aid of the numerically based effective
medium method. By making use of the low locally resonant frequency of the EMM plate, the EMM plate is carefully
designed with different dimensions to attain high effective mass densities. The effective mass density can be assumed to
dominate the change of wave velocity and propagation direction in the EMM plate. An effective mass density profile is
then employed along the transverse direction of wave propagation to achieve focusing. Finally, numerical simulation
with finite element method (FEM) is utilized to investigate the focusing phenomenon of the A<sub>0</sub> mode Lamb waves at 30
kHz and the out-of-plane displacement response beyond the EMM region. Numerical simulation results have shown that
focusing the low frequency A<sub>0</sub> mode Lamb waves using EMMs is feasible. The focusing may have potential applications
in structural health monitoring by manipulating Lamb waves through controlling and focusing Lamb waves to any
arbitrary location of the plate with amplified displacement and yet largely retained five-peaked toneburst waveform.

In this paper a novel electromagnetic vibration type energy harvester which uses a diamagnetic levitation system is
conceptualized, designed, fabricated, and tested. The harvester uses two diamagnetic plates made of pyrolytic graphite
between which a cylindrical magnet levitates passively. Two archimedean spiral coils are placed in grooves which are
engraved in the pyrolytic graphite plates, used to convert the mechanical energy into electrical energy efficiently. The
geometric configurations of coils are selected based on the field distribution of the magnet to enhance the efficiency of
the harvester. A thorough theoretical analysis is done to compare with the experiment results. At an input power of
103.45 &mu;W and at a frequency of 2.7 Hz, the harvester generated a power of 0.744 &mu;W at an efficiency of 0.72 %. Both
theoretical and experimental results show that this new energy harvesting system is efficient and can capture low
frequency broadband spectra.

This paper presents a novel fabrication method of PZT micro-fibers using activated carbon template with the aim of
manufacturing PZT/epoxy 1-3 composites. Porous carbon was first prepared by chemical activation technology. The pore
diameter formed in an activated carbon template is of several microns and lengths are up to several millimeters. These
pores provide a basic platform to grow PZT fibers inside. Then the carbon template is removed at high calcination
temperatures to form PZT micro-fibers. Subsequently, thermo-gravimetric analysis (TG) and differential scanning
calorimetry (DSC) were performed to analyze the process of removing the template as temperature changing. For
manufacturing 1-3 piezo-composites, the PZT fibers were carefully aligned in one direction and infiltrated by epoxy resin.
Based on the observation from X-ray diffraction (XRD) the fibers show a pure pervoskite phase at low sintering
temperature of 950&deg;C. The fibers embedded orderly in the epoxy matrix are smoothly distributed and straightened which
were observed using a scanning electron microscopy (SEM). The diameter of fibers is around several microns with the
length up to a few millimeters, matching well with pores in the template. The new micro-fiber composite material can be
potentially used in a sensor with high directivity in structural health monitoring.

With increasing application of composite materials, real time monitoring of composite structures becomes vital for
maintenance purpose as well as prevention of catastrophic failure. It has been reported that carbon nanotubes (CNTs)
have excellent piezoresistive properties, which may enable a new generation of sensors in nano or micro scales. We
report here a novel prototype of carbon nanotube yarn sensors with excellent repeatability and stability for in-situ
structural health monitoring. The CNT yarn is spun directly from CNT arrays, and its electrical resistance increases
linearly with tensile strain, which makes it an ideal strain sensor. Importantly, it shows repeatable piezoresistive behavior
under repetitive straining and unloading. Yarn sensors show stable resistances at temperatures ranging from -196&deg; to
110&deg;. Neat yarn sensors are also embedded into resin to monitor the loading conditions of the composites. With
multiple yarn sensor elements aligned in the composite, the crack initiation and propagation could be monitored. Yarn
sensors could be easily incorporated into composite structures with minimal invasiveness and weight penalty to enable
the structure has self-sensing capabilities.

A comprehensive physics-based model for predicting the performance of the miniature wind turbine (MWT) for power
wireless sensor systems was proposed in this paper. An approximation of the power coefficient of the turbine rotor was
made after the turbine rotor performance was measured. Incorporation of the approximation with the equivalent circuit
model which was proposed according to the principles of the MWT, the overall system performance of the MWT was
predicted. To demonstrate the prediction, the MWT system comprised of a 7.6 cm thorgren plastic propeller as turbine
rotor and a DC motor as generator was designed and its performance was tested experimentally. The predicted output
voltage, power and system efficiency are matched well with the tested results, which imply that this study holds promise
in estimating and optimizing the performance of the MWT.

A defect detection technique based on time-reversal concept is proposed to detect and locate the defects in a plate
structure. Time-reversal imaging method is widely use as an advanced, robust data processing and imaging technique in
structure health monitoring to detect the defects. Physically, the time reversed signal will retrace its original path
precisely, which means that the signals will be refocused back on the source and defects after we record, time reverse
and back propagate the wave signal experimentally or numerically. In this paper, a distributed actuator/sensor network is
placed on a square homogeneous plate to generate and collect the wave signals in the plate. The time-reversal technique
is then used to interpret the physical meaning of the recorded data and image the defects in the plate. Computer
simulations are presented to illustrate the feasibility of the technique in this paper.

Materials State Awareness (MSA) goes beyond traditional NDE and SHM in its challenge to characterize the current
state of material damage before the onset of macro-damage such as cracks. A highly reliable, minimally invasive system
for MSA of Aerospace Structures, Naval structures as well as next generation space systems is critically needed.
Development of such a system will require a reliable SHM system that can detect the onset of damage well before the
flaw grows to a critical size. Therefore, it is important to develop an integrated SHM system that not only detects macroscale
damages in the structures but also provides an early indication of flaw precursors and microdamages. The early
warning for flaw precursors and their evolution provided by an SHM system can then be used to define remedial
strategies before the structural damage leads to failure, and significantly improve the safety and reliability of the
structures. Thus, in this article a preliminary concept of developing the Hybrid Distributed Sensor Network Integrated
with Self-learning Symbiotic Diagnostic Algorithms and Models to accurately and reliably detect the precursors to
damages that occur to the structure are discussed. Experiments conducted in a laboratory environment shows potential of
the proposed technique.

In this paper, a miniature wind turbine (MWT) system composed of commercially available off-the-shelf components
was designed and tested for harvesting energy from ambient airflow to power wireless sensors. To make MWT operate
at very low air flow rates, a 7.6 cm thorgren plastic Propeller blade was adopted as the wind turbine blade. A sub watt
brushless DC motor was used as generator. To predict the performance of the MWT, an equivalent circuit model was
employed for analyzing the output power and the net efficiency of the MWT system. In theory, the maximum net
efficiency 14.8% of the MWT system was predicted. Experimental output power of the MWT versus resistive loads
ranging from 5 ohms to 500 ohms under wind speeds from 3 m/s to 4.5 m/s correlates well with those from the predicted
model, which means that the equivalent circuit model provides a guideline for optimizing the performance of the MWT
and can be used for fulfilling the design requirements by selecting specific components for powering wireless sensors.

In this study, an optimal vibration-based energy harvesting system using magnetostrictive material (MsM) has been
designed to power the Wireless Intelligent Sensor Platform (WISP), developed at North Carolina State University. A
linear MsM energy harvesting device has been modeled and optimized to maximize the power output. The effects of
number of MsM layers and glue layers, and load matching on the output power of the MsM energy harvester have been
analyzed. From the measurement, the open circuit voltage can reach 1.5 V when the MsM cantilever beam operates at
the 2nd natural frequency 324 Hz. The AC output power is 0.97 mW, giving power density 279 &mu;W/cm<sup>3</sup>. Since the MsM
device has low open circuit output voltage characteristics, a full-wave quadrupler has been designed to boost the rectified
output voltage. To deliver the maximum output power to the load, a complex conjugate impedance matching between the
load and the MsM device has been implemented using a discontinuous conduction mode (DCM) buck-boost converter.
The maximum output power after the voltage quadrupler is now 705 &mu;W and power density reduces to 202.4 &mu;W/cm<sup>3</sup>,
which is comparable to the piezoelectric energy harvesters given in the literature. The output power delivered to a
lithium rechargeable battery is around 630 &mu;W, independent of the load resistance.

Propagation of torsional elastic waves in the clad core is addressed in this paper. The shear velocity of the core is slightly
smaller than that in the cladding. Core with cladding of different finite thickness and infinite thickness is investigated.
Two types of modes, guided and leaky modes are examined with the discussion of motion in waveguide. Phase, group,
and energy velocities, cutoff frequencies are analyzed and the results of first three modes are presented. The change of
dispersion curves due to variation of thickness of cladding is discussed and it is found that when the thickness increases
the results of finite clad core will approach those of infinite clad core in guided mode, but not in leaky mode. Below
cutoff frequencies the wavenumber becomes complex in infinite clad core, while it is pure imaginary in finite clad cores.
The group and energy velocity are presented and in leaky mode the group velocity becomes abnormal, while the energy
velocity is physically meaningful.

Image segmentation for quantifying damage based on Bayesian updating scheme is proposed for diagnosis and prognosis
in structural health monitoring. This scheme enables taking into account the prior information of the state of the
structures, such as spatial constraints and image smoothness. Bayes' law is employed to update the segmentation with
the spatial constraint described as Markov Random Field and the current observed image acting as a likelihood function.
Segmentation results demonstrate that the proposed algorithm holds promise of searching a crack area in the SHM image
and focusing on the real damage area by eliminating the pseudo-shadow area. Thus more precise crack estimation can be
obtained than the conventional K-means segmentation by shrinking the fuzzy tails which often exist on both sides of the
crack tips.

A new consistent higher-order plate theory is developed for composites with the aim of accurately and efficiently
modeling multiple higher-order Lamb waves over a higher frequency range. The dispersion relations based on this
theory that can be analytically determined comprise five symmetric and six anti-symmetric wave modes. Computational
procedures for phase and group velocities are discussed. Meanwhile, characteristic wave curves including velocity,
slowness, and wave curves are introduced to investigate the dispersive and anisotropic behavior of Lamb wave
propagation in composites. From numerical results of Lamb waves in both lamina and symmetric laminate, it shows that
the higher-order plate theory not only gives good agreement with three-dimensional (3-D) elasticity theory over a wide
high frequency range, but also provides a more robust method than 3-D elasticity theory. This study demonstrates a
feasibility of using the proposed theory for realizing near real-time Structural Health Monitoring for composites at a
higher frequency range.

It is well known that vibration-based damage detection methods lack sensitivity in modal frequencies to
small changes in mass, stiffness, and damping parameters induced by damage. To circumvent this deficiency, in this paper a scheme through feedback control together with coherence method is first
employed to enhance sensitivity the occurrence and location of damage through few of the lower natural frequencies. This sensitivity enhancement is based on pole placement from a single-point feedback control to compute the control gains. Relying on <i>a priori</i> knowledge of how certain damage scenarios affect modal
properties, a coherence method correlates measured and predicted modal frequency shifts for a given set of damage scenarios to locate the damage. Numerical results show that the method with closed-loop control increases both the accuracy of locating damage and the ability to tolerate environmental noise. Subsequent to the fine sensitivity to locating the damage position, restoration to original dynamic structural performance in terms of few of the lower natural frequencies of the damaged structure is conducted by the feedback control using distributed actuation surrounding the damage area (multi-point control). Simulation results show that the dynamic characteristics of damaged structures can be successfully restored by applying distributed actuation that can be induced from the voltage by the distributed piezoelectric actuators.

A new class of vibrational energy harvester based on Magnetostrictive material (MsM) Metglas 2605SC is deigned,
developed, and tested in building practical energy harvesting wireless sensor networks. Compared to piezoelectric
material, Metglas 2605SC offers advantages including ultra-high energy conversion efficiency, high power density,
longer life cycles without depolarization issue, and flexibility to operate in strong ambient vibrations. To enhance the
energy conversion efficiency and shrink the size of the harvester, Metglas is annealed in the direction normal to the axial
strain direction without the need of electromagnet for applying bias (static) magnetic field. To seamlessly integrate with
a newly developed wireless sensor at NC State<sup>1</sup>, a prototype design for the MsM harvester is proposed. An analytical
model is developed for the harvesting using an equivalent electromechanical circuit. The model resulting in achievable
output performances of the harvester powering a resistive load and charging a capacitive energy storage device,
respectively, is quantitatively derived. An energy harvesting module, which powers a wireless sensor, stores excess
energy in an ultracapacitor is designed on a printed circuit board (PCB) with dimension 25mm x 35mm. The main
functionalities of the circuit include a voltage quadrupler, a 3F ultracapacitor, and a smart regulator. The output DC
voltage from the PCB can be adjusted within 2.0~5.5V. In experiments, the maximum output power and power density
on the resistor can reach 200 &mgr;W and 900 &mgr;W/cm<sup>3</sup>, respectively. For a working prototype, the average power and power
density during charging the ultracapacitor can achieve 576 &mgr;W and 606 &mgr;W/cm<sup>3</sup> respectively, which are much higher
than those of most piezo-based harvesters.

This paper focuses on the existence of <i>higher-order </i>Lamb wave modes that can be observed from piezoelectric sensors by the excitation of ultrasonic frequencies from piezoelectric actuators. Using three-dimensional (3-D) elasticity theory, the exact dispersion relations governed by transcendental equations are numerically solved for an infinite number of possible wave modes. For symmetric laminates, a robust method by imposing boundary conditions on mid-plane and top surface is developed to separate wave modes. Then both phase and group velocity dispersions of Lamb waves in composites are obtained. Meanwhile three characteristic wave curves including velocity, slowness, and wave curves are introduced to analyze the angular dependency of Lamb wave propagation at a given frequency. In the experiments, two surface-mounted piezoelectric actuators are operated corporately to excite either symmetric or anti-symmetric wave modes with narrow banded excitation signals, and a Gabor wavelet transform is used to extract group velocities from arrival times of Lamb wave received by a piezoelectric sensor. In comparison with the results from the theory and experiment, it is confirmed that the higher-order Lamb waves can be excited from piezoelectric actuators and the measured group velocities agree well with those from 3-D elasticity theory.

In this paper a pre-stack reverse-time migration concept of signal processing techniques is developed and adapted to guided-wave propagation in composite structure for multi-damage imaging by experimental studies. An anisotropic laminated composite plate with a surface-mounted linear piezoelectric ceramic (PZT) disk array is studied as an example. At first, Mindlin Plate Theory is used to model Lamb waves propagating in laminates. The group velocities of flexural waves are also derived from dispersion relations and validated by experiments. Then reconstruct the response wave fields with reflected data collected by the linear PZT array. Reverse-time migration technique is then performed to back-propagate the reflected energy to the damages using a two-dimensional explicit finite difference algorithm and damages are imaged. Stacking these images together gets the final image of multiple damages. The experimental results show that the pre-stack migration method is hopeful for damage detection in composite structures.

As wireless sensor has emerged as a promising technology in recent years, active sensing which integrates actuation capability in a wireless sensor unit for detecting localized damage has been brought into sight in structural health monitoring. However, the inefficiency of conventional energy conversion system is a major constraint for the utilization of reactive actuator, such as piezoelectric, in the wireless sensor unit since power source is limited. This paper proposes a highly efficient low power switching amplifier to drive piezoelectric disc in the high frequency for wireless sensor applications.

In this paper, a wavelet-based built-in damage detection and identification algorithm for carbon fiber reinforced polymer (CFRP) laminates is proposed. Lamb waves propagating in laminates are first modeled analytically using higher-order plate theory and compared them with experimental results in terms of group velocity. Distributed piezoelectric transducers are used to generate and monitor the fundamental ultrasonic Lamb waves in the laminates with narrowband frequencies. A signal processing scheme based on wavelet analysis is applied on the sensor signals to extract the group velocity of the wave propagating in the laminates. Combined with the theoretically computed wave velocity, a genetic algorithms (GA) optimization technique is employed to identify the location and size of the damage. The applicability of this proposed method to detect and size the damage is demonstrated by experimental studies on a composite plate with simulated delamination damages.

Conventional wireless sensors for structural health monitoring do not accommodate the need of high frequency data acquisition. Lack of development of this type of wireless smart sensors will without doubt hinder the applications of active diagnostic methods, normally used in local damage interrogation. In this paper, a novel wireless smart sensor design, using FPGA as co-controller with ultra sensing capability, is presented. The development and some outstanding issues of the sensor are discussed in detail, and a preliminary experimental result is given to verify the effectiveness of this wireless smart sensor design.

A theory is developed that incorporates the piezoelectric effect into slewing flexible composite materials using classical laminate theory. Using the piezoelectric material as a modal sensors allows for placement of all of the poles of the system without the need for a state observer design. Pole placement is used on a numeric example involving a graphite epoxy beam and a DC motor. Critical damping of both the motor and beam are achieved.

My Library

You currently do not have any folders to save your paper to! Create a new folder below.

Keywords/Phrases

Keywords

in

Remove

in

Remove

in

Remove

+ Add another field

Search In:

Proceedings

Volume

Journals +

Volume

Issue

Page

Advanced PhotonicsJournal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews